Sixteen infants less than 2 years of age with apparently idiopathic hypopituitarism were studied. At birth, 11 of 16 patients (69%) had subnormal length associated with relative adiposity and 10 of 16 (62%) showed significant deterioration in length deficiency from birth onwards. These findings suggest that: (a) growth hormone deficiency, in a number of patients, had started well before delivery; (b) growth hormones may play a role in intrauterine growth; and (c) growth hormone may also be involved in early postnatal growth. Magnetic resonance imaging in these patients was very similar to that described in hypopituitarism of later onset. This suggests that even in the latter case, hypopituitarism may have a prenatal onset. Finally, the severity of growth failure and the coexistence of other hypopituitary symptoms at the time of diagnosis in 31% of our patients indicate that early clinical screening of hypopituitarism is possible.